1. Field of the Invention
The present invention relates to a motor control device and a method for detecting out-of-step and particularly to a motor control device for controlling a stepping motor and a method for detecting out-of-step in the stepping motor.
2. Description of the Related Art
A stepping motor is a motor which is driven in synchronous with pulsed power. The stepping motor is able to realize an accurate positioning control with a simple circuit configuration. For example, the stepping motor is used as an actuator for an air conditioning unit installed in a vehicle. In a usage of the stepping motor, it sometimes happen that synchronization between an input pulse signal and rotation of a motor is lost and thus the control of the stepping motor is disturbed when an excessive load or a sudden change in speed occurs. This state is referred to as “out-of-step.”
Techniques for detecting the out-of-step of the stepping motor are disclosed in documents such as EP 1460757 A1, JP-A-2009-261045 and JP-A-2012-016221.
EP 1460757 A1 discloses an out-of-step detection technique to provide a stop period in a period of each step unit in a control signal of a stepping motor. In the stop period, supply of control signal is stopped only by a time short enough to not affect the rotation of the stepping motor. The technique disclosed in EP 1460757 A1 detects an out-of-step of the stepping motor by measuring a back electromotive voltage induced in a coil during the stop period.
JP-A-2009-261045 discloses a technique which allows a stepping motor to be used in the maximum torque and detects an out-of-step while the stepping motor is driven by the technique disclosed in EP 1460757 A1. In the solving means section of the Abstract of JP-A-2009-261045, a configuration is disclosed in which a means 5 for supplying a control current to coils 4 of each phase so as to drive a stepping motor 1 and a means for separately measuring back electromotive voltages (VA, VB) induced in each of the coils of each phase are used to stop the control current of the coils of the each phase in the order of each phase by a time t that is short enough not affecting the rotation of the stepping motor at a predetermined time (T1 to T6) during one step stepping of a rotor 3 of the stepping motor and to measure the back electromotive voltages during the stop. In this way, the out-of-step is detected when the measurement results in at least one phase coil meet a predetermined out-of-step determination criterion.
FIGS. 8A and 8B are explanatory views showing an operation of detecting an out-of-step of a motor according to a comparative example, which is disclosed in JP-A-2009-261045.
FIG. 8A is a time chart showing an A-phase control current. A circle shown on the right is an enlarged view of a part near time T1.
Time T1 to T6 indicates each time during one step stepping of a rotor of a stepping motor according to the comparative example.
Macroscopically, the A-phase control current flows in a positive direction at time 0 to T1, flows in a reverse direction at time T1 to T3 and flows in a positive direction at time T3 to T5.
Microscopically, the A-phase control current becomes zero over a time interval t near time T1. This time interval is often referred to as “energizing stop period.” The motor control device detects an out-of-step by measuring a back electromotive voltage of the A-phase coil during the energizing stop period.
FIG. 8B is a time chart showing a B-phase control current. A circle shown on the right is an enlarged view of a part near time T4.
Time T1 to T6 indicates each time during 1 step stepping of the rotor of the stepping motor according to the comparative example.
Macroscopically, the B-phase control current flows in a positive direction at time 0 to T2, flows in a reverse direction at time T2 to T4 and flows in a positive direction at time T4 to T6.
Microscopically, the B-phase control current becomes zero over a time interval t near time T4. The motor control device detects an out-of-step by measuring a back electromotive voltage of the B-phase coil during the energizing stop period.
FIG. 9 is an enlarged view of a part near time T1 in FIGS. 8A and 8B. In a waveform diagram of each phase control current, a solid line indicates a control current and a dashed line indicates a target current. In the comparative example, both the A-phase coil and the B-phase coil are controlled while the control current flowing through the A-phase coil and the B-phase coil is set to the target current value.
As shown in FIG. 9, when the A-phase control current is a positive value, a pulse voltage having a predetermined duty is applied to the A-phase coil.
During a time period in which the A-phase control current is zero, which corresponds to the energizing stop period, a back electromotive voltage is generated in the A-phase coil. An out-of-step of the stepping motor can be detected by the back electromotive voltage. At this time, a pulse voltage having a predetermined duty is applied to the B-phase coil. As shown by an ellipse labeled “D” in FIG. 9, noise generated in the on-off switching of energizing executed by a pulse width modulation (hereinafter, referred to as PWM) is superimposed on a waveform of a back electromotive voltage of the A-phase coil during the energizing stop.
In the object section of the Abstract of JP-A-2012-016221, it is disclosed that the detection precision of an out-of-step of a stepping motor is improved. In the solving means section of the Abstract of JP-A-2012-016221, a motor control device 10 is disclosed in which a controller applies a pulse voltage subjected to pulse width modulation to each of coils of a stepping motor 20. The stepping motor 20 includes at least two phase coils, through which coil currents having different phases flow, respectively, and a rotor configured to rotate by periodically switching a phase of the coil current flowing through each phase coil. A back electromotive voltage measuring unit 126 provides a stop period where the application of the pulse voltage to the coil is temporarily stopped when a direction of the coil current flowing through one phase coil is switched and measures a back electromotive voltage induced in the one phase coil during the stop period. An out-of-step detecting unit detects an out-of-step of the stepping motor 20 when the measured back electromotive voltage meets a predetermined out-of-step determination criterion. During the stop period, a voltage controller sets, to a fixed voltage, all coils other than the coil where the back electromotive voltage is measured